Comparison of Bending Stiffness of Cross-Laminated Solid Timber Derived by Modal Analysis of Full Panels and by Bending Tests of Strip-Shaped Specimens
The design of cross-laminated solid timber (CLT) as load-bearing plates is mainly governed by serviceability criterions like maximal deflection and susceptibility to vibration. Hence, predicting the respective behavior of such plates requires accurate information about their elastic properties. According to product standards, the bending stiffness of CLT has to be assessed from 4-point bending tests of strip-shaped specimens, cut from the CLT panels. By comparing elastic properties of CLT derived by means of modal analysis of full panels with the results of bending tests on 100 mm and 300 mm wide strip-shaped specimens it is shown, that by testing single 100 mm wide strip-shaped specimens bending stiffness of full panels cannot be assessed correctly, whereas single 300 mm wide strips or averages of 5 to 6 100 mm wide strip-shaped specimens lead to acceptable results. Hence, strip-shaped specimens should only be used in the course of factory quality control or when assessing the bending stiffness of parts of CLT panels used as beam-like load-bearing elements but not to derive bending stiffness of gross CLT panels. Verification by carrying out static bending tests of gross CLT panels under different loading situations showed that alternatively to tests on strip-shaped specimens or estimations with the compound theory, the overall stiffness properties of CLT can be derived directly by a modal analysis of full-size panels.
Cross laminated timber (CLT) has the potential to play a major role in timber construction as floor and wall systems. In order to meet specific design needs and to make the use of CLT more effective, property evaluation of individual CLT panels is desirable. Static tests are time-consuming and therefore costly, and for massive products such as CLT practically impossible to implement. Modal testing offers a fast and more practical tool for the property evaluation of CLT and timber panels in general. This paper presents a comparison of different boundary conditions in modal testing in terms of accuracy, calculation effort and practicality. Single-layer timber panels as well as scaled CLT panels were fabricated. Three elastic properties of the panels were evaluated using modal testing methods with different boundary conditions (BCs). The results were compared with results from static test.
The interlaminar shear stresses of the three-layer, five-layer, and seven-layer cross laminated timber (CLT) and those of the oriented laminated beams were calculated according to Hooke's law and the differential relationship between the beam bending moment and shear force. The interlaminar and maximum shear stresses of the CLT beam are related to the number of CLT layers and to the elastic modulus ratio EL/ET (or EL/ER) of the parallel and perpendicular layers. The interlaminar shear strength of the Hemlock CLT was positively correlated with the elastic modulus of its parallel layer. The results showed that the CLT short-span beams had three failure modes when subjected to a three-point bending test, namely perpendicular layer rolling shear failure, CLT interlaminar shear failure, and parallel layer bending failure. The shear stress of the oriented laminated beam followed a parabolic distribution along the height of the section, while the shear stress of the orthogonally laminated beams tended to be balanced, rather than parabolically distributed along the height of section. The short beam three-point bending method was able to effectively test the interlaminar shear strength of CLT due to its stable and readable load.
With the advent of mass timber panels and the development of mid- to high-rise wood constructions, the renaissance of wood construction is underway from Europe to North America and throughout the world. Engineered wood-based panel products, especially mass timber panels, play an important role in the evolution of wood construction. Elastic properties are not only fundamental mechanical properties for structural design but also important indicators for quality control purposes. Accurate measurement of the global elastic properties of full-size panels is critical for their applications as load-bearing building components. An efficient and reliable non-destructive technique is required for the purposes both of characterizing elastic properties and of grading engineered wood-based panel products in the production line before processing for all kinds of structural applications. In this study, two vibrational non-destructive techniques employing modal testing for natural frequencies and other modal parameters were developed for simultaneous measurement of elastic constants of engineered wood-based panels. Both vibrational methods adopted modal testing of a rectangular plate with the boundary condition of a pair of opposite edges in the width direction simply supported and the other pair free. Compared with the elastic constant values by conventional static tests, both vibrational methods generally showed close agreement. The first method was developed for measuring the moduli of elasticity in both major and minor strength directions and the in-plane shear modulus of a panel based on free transverse vibration of rectangular thin orthotropic plates. A simplified modal testing procedure together with frequency identification methodology based on sensitivity analysis and an iterative algorithm were proposed as the means of achieving an efficient and reliable measurement with three and/ or four sensitive natural frequencies from only three impacts. The method was first verified with standard static test values in laboratory for full-size cross laminated timber, oriented strand board and medium density fibreboard. Then, 55 full-size cross laminated timber panels with different characteristics and from three manufacturers were tested in factory environments. The results showed that non-edge bonding and gap size had a negative effect on both Ey and Gxy and led to a large variation compared with edge bonded panels as well as with their corresponding prediction models (i.e., k-method, gamma method and shear analogy method). The second vibrational method was developed for determination of effective bending and shear stiffness values based on Mindlin plate theory with an exact frequency solution and a genetic algorithm for the inverse problem. The results showed that the transverse shear moduli of cross laminated timber panels can be accurately determined with proper shear correction factors and were verified by planar shear test values. According to an in-depth comparative study, the first vibrational method shows great potential for future development of a standard testing method and on-line quality control over other existing vibrational methods in terms of setup implementation, frequency identification, accuracy and the calculation efforts required. The second vibrational method is suggested for engineered wood-based panels with small transverse shear moduli and/ or small length/ width to thickness ratio. Both methods are deemed to be applicable to all kinds of composite plates.
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